RF power transistor devices are generally known for use as signal amplifiers in wireless communication applications. As demand for wireless communication applications has increased, the operating frequency for wireless networks has also increased. Operating frequencies now span well into the gigahertz range.
Variables in individual transistor elements contributed by inherent parasitics pose a big challenge for mass production of RF power transistors problematic. Transistor devices naturally vary as to input capacitance, gain and phase shift. Particular transistor devices are preliminarily characterized over a range of expected operating frequencies and voltages. In particular, parasitic elements of an RF power transistor cause inherent non-linearities to the respective transistor. RF power amplifiers for 3rd generation base stations are designed to operate with an approximately 4 MHz wide spread spectrum Wideband Code Division Multiple Access (W-CDMA) format and may be used with one, or any combination of, two to four carriers. In any case, the maximum expected instantaneous spread-spectrum bandwidth is about 20 MHz.
This coincides with the 20 MHz wide sub-bands of the Universal Mobile Telecommunication System (UMTS) or Personal Communication Services (PCS) spectrums. These spectra are 60 MHz wide, to facilitate different service providers operating simultaneously in their respective allocated sub-bands.
In order to speed up the implementation of these 3rd generation networks, there is an increased interest among operators in sharing “common” infrastructure elements, including the base stations with their RF power amplifiers and antennae. Such a solution has already been allowed by European regulatory agencies.
In such a scenario, one can envision operation of the base station power amplifier with signals present in different sub-bands simultaneously. Consequently, although each operator is confined to his allocation, the actual bandwidth of the spread-spectrum signals may reach 60 MHz.
The non-linearities of an RF power transistor, as mentioned above, are enhanced by the effect of internal and external matching networks designed to optimize power transfer from the transistor. This phenomenon is detrimental to the wideband linear performance of the amplifier.
The bias circuitry used to deliver the DC power to the transistor, in addition to the elements to suppress the lower frequency spurious generation, is known to interact with the RF matching circuit and affect the operating/video bandwidth of the device, manifesting itself in degradation of both power transfer and the wideband linearity of the device. Such a matching circuit is known, for example, from U.S. Pat. No. 6,734,728 the entire content of which is hereby incorporated by reference.
U.S. Pat. No. 6,734,728 shows an example for a RF power transistor with a shunt network. Such implementation resolves some of the problems associated with the wideband performance of an RF power transistor. Such a method improves the video bandwidth of the transistor from about 15-20 MHz up to about 40-45 MHz.